With the increasing demands on wind turbines to contribute to grid stabilization in the event of a drop in frequency, there are also growing demands by the grid operators concerning the maximum drop in power following provision of primary control power by the wind turbine. Various methods for providing primary control power are known from the prior art, in which the required primary control power is obtained from the kinetic energy of the rotor.
Known from WO 2011/124696 is a closed-loop control system for wind turbines that provides adequate primary control power even in the case of unsteady wind speeds. In this case, an additional closed-loop control system is provided, which has an input for a desired additional power, and which is designed to generate therefrom a rotational-speed change signal, taking account of a rotor moment of inertia, and to output an output signal that is added to the setpoint rotational-speed signal via a logic element.
A rotational-speed change signal is understood to mean the change in rotational speed over time that results from energy being removed from the inertia mass of the wind rotor. This energy corresponds to the difference between the kinetic energy stored in the inertia mass before and after the change in rotational speed is taken into account. The desired additional power is made available on the basis of the rotational-speed change signal. In this case, the power delivered to the grid by the wind turbine is not controlled by closed-loop control, but instead the energy removed from the inertia mass of the wind rotor is controlled by open-loop control. After the infeeding of the additional power is completed, there is again a changeover to normal operation. This is not effected abruptly, however, but gradually, in order to avoid instabilities in the mechanical and electrical system of the wind turbine.